feat: Implement member contribution modal with full Soroban transaction flow

[derekliutao-cmd]

Summary

Closes #12

Implements src/components/groups/contribute-modal.tsx — the full member contribution flow with real Soroban transaction submission.

Changes

src/components/groups/contribute-modal.tsx (new file, 394 lines):

Transaction Flow

1. Amount input (USDC, validated against group.rules.minContribution)
2. Period selector (auto-populated with current cycle YYYY-QN, editable)
3. Wallet balance displayed before submitting
4. On submit:
   • Builds Soroban transaction calling treasury.contribute(member, amount, period)
   • Simulates transaction first (catches errors before signing)
   • Signs with useWallet().signTransaction()
   • Submits via SorobanRpc.Server.sendTransaction()
   • Polls for transaction result (up to 30 attempts × 3s)

Transaction States

• idle → Form editable, submit enabled
• signing → Waiting for wallet signature, form disabled
• pending → Transaction submitted, polling for confirmation
• success → Tx hash + Stellar Explorer link shown, queries invalidated
• error → Error message + Try Again button

Acceptance Criteria

• Full Soroban transaction built correctly (not simulated — actually submitted)
• Wallet balance displayed before submitting
• Amount validated against group minimum contribution
• All 4 transaction states handled: idle / signing / pending / success / error
• Tx hash displayed on success with explorer link
• Group balance and member history refresh after success (queryClient.invalidateQueries)

Error Handling

• Simulation failures
• Wallet rejection (rejected/denied/cancelled)
• Network errors
• On-chain transaction failures
• Confirmation timeout

Uses existing dependencies (@radix-ui/react-dialog, @stellar/stellar-sdk, @tanstack/react-query, lucide-react).

[derekliutao-cmd]

Submitted PR #35 with full Soroban transaction flow. All acceptance criteria met. Happy to address any feedback!